A semiconductor acceleration sensor is conventionally known to detect an acceleration based on a movement of a weight part relative to a fixed part when the acceleration is applied thereto. This acceleration sensor has a weight part which is a movable part provided on an end of a cantilever-like bending portion (thin beam part) extending from the fixed part, and has such a structure that causes the bending portion to bend and the weight part to move when an acceleration is applied thereto, and that electrically detects the change. As described in Japanese Laid-open Patent Publication 2000-338124 and Japanese Laid-open Patent Publication Hei 7-159432, examples of the type for electrically detecting movement of the weight part include, for example: acceleration sensors that form a gauge resistor on a bending portion connecting the weight part, and measure a change in a resistance value of the gauge resistor so as to detect a bending of the bending portion; and those that form the weight part itself into an electrode, and detect a change in electrostatic capacity caused by movement of the weight part to change the distance from a fixed electrode part.
This kind of acceleration sensors are formed to allow the volume (weight) of the weight part to be large, and the bending portion to be as thin as possible, in order to achieve smooth movement of the weight part when an acceleration is applied. This has led to a risk that the bending portion may break, making it impossible to detect the acceleration, when the weight part suddenly moves, or moves beyond a normal movement range.
Thus, in order to prevent the above problem, an acceleration sensor has been developed which has a stopper formed in a space around the weight part to limit the movement of the weight part. For example, in the acceleration sensor described in the above Patent Publication 1, stoppers are formed at upper and lower positions in a space for the weight part to move so that the weight part is prevented from moving beyond a predetermined amount in an up and down direction which is the primary direction to detect the acceleration. Furthermore, a stopper is also formed beside the weight part so that the weight part hardly moves when a side impact is applied, thereby preventing the bending portion from breaking.
On the other hand, there is an acceleration sensor in which the length of beam parts (bending portions) connecting a fixed part to a weight part is elongated so as to increase the sensitivity of the acceleration sensor. As shown in FIG. 16, this sensor 50 has beam parts 51 formed in L-shape having proximal ends 51a at two opposite sides of a weight part 52 and extending along two adjacent sides of the weight part 52 so as to form a structure such that two beam parts 51 surround the perimeter of the weight part 52. In this sensor 50, the beam parts 51 bend and the weight part 52 moves upward or downward when an acceleration is applied in an up and down direction (direction to penetrate the sheet surface of FIG. 16). Now, this acceleration sensor 50 is advantageous in that the lengths of the beam parts 51 can be formed to be sufficiently long, thereby making it possible to considerably increase the sensitivity of detecting acceleration. However, it is not easy to form a stopper such as shown in the above Patent Publication 1 to limit a side movement of the weight part 52, and there has been a risk that the beam parts 51 may consequently break due to an excessive movement of the weight part 52 in a horizontal direction.
Furthermore, in an acceleration sensor having a structure with the two beam parts 51 surrounding the perimeter of the weight part 52 as described above, in contrast to a weight part connected by a cantilever-like bending portion as described in the Patent Publications 1 and 2, it is possible for the weight part 52 to move in any directions (front-back and left-right direction) in the horizontal direction. It has been difficult to effectively limit the movements in any directions by a simple stopper.